The present invention relates to the preparation of polyolefins. The invention relates in particular to a process according to the preamble of claim 1 for the treatment of a gas stream obtained from the preparation of polyolefins and containing unreacted compounds. The said streams are obtained when xcex1-olefins are polymerized in the presence of a catalyst in at least one reactor in order to form a reaction product which contains polyolefin. The reactor effluent is in this case directed to a separator, where the gas stream containing unreacted compounds is separated and the polymer product is recovered. The substantially uncondensed compounds are removed from the gas stream obtained from the separator and are directed to further treatment.
Olefins are polymerized to polyolefins in the presence of, for example, Ziegler-Natta type catalysts or metallocene catalysts. Loop reactors, fluidized-bed reactors, stirred gas-phase reactors, and fluidized-bed reactors provided with stirring members are examples of the reactor types used. The reactor contains a monomer (typically ethylene or propylene) and possibly a comonomer (C2-C10 olefins or diolefins, preferably ethylene, propylene, 1-butene or hexene) and hydrogen, which controls the molar mass distribution. The reactor may also contain high concentrations of a substance inert in terms of the reaction (typically nitrogen or propane).
It is typical of all of the reactors mentioned above that, together with the polymer obtained as a product there emerges from the reactor unreacted monomer, comonomer and often diluent (a substance inert in terms of the reaction).
The gases coming from the product separators of the reactors are mainly returned to the process; some are directed to the flare or for fuel gas. The gases may also be directed to distillation, if it is desired to divide the monomer, the comonomer and the diluent into separate streams for reuse. The problem involved in separation by distillation is that in the gas stream exiting from the reactors there are always also present byproducts of the reaction, heavy oligomers, and light hydrocarbons and hydrogen which have entered with the feed. These byproducts complicate polymerization in mainly two ways:
When they end up in the said distillation products (monomer, comonomer and diluent) they are recycled to the reactors, whereby their concentration in the reactor gradually increases. If such a situation continues for too long, the operation of the process is complicated to such an extent that it has to be interrupted and shut down. This in turn means a very great financial loss.
The accumulating inert compounds are as a rule light components. Since the separation of the gases is carried out by distillation, the temperature of the overhead product of the distillation column has to be lowered owing to the light components. In this case this leads to the situation that in the condenser of at least one overhead product of the distillation column it is not possible to use air or water, which are the most common and the least expensive condensing media.
In a typical plant producing polyolefins, a number of products having different comonomer concentrations and different molar mass distributions are prepared. These grade change situations regularly also change the composition of the gas exiting together with the product. In the following there are given a few principles of grade change situations:
1. When a shift is made to products differing in comonomer concentrations or when it is desired to change the monomer concentration in a circulation gas stream which contains inerts, grade change typically does not cause great problems. If it is desired to lower the concentration of the reactive component concerned, its feed is lowered or discontinued, and the reaction rather rapidly consumes the excess. If it is desired to increase the component concentrations concerned, the change usually has to be made relatively calmly so that a rapid change of the reaction conditions should not cause problems. Furthermore, the typical changes in the comonomer or monomer concentrations are rather small, and thus the changes can be carried out relatively smoothly. The hydrogen amounts used are typically rather small, and thus the increasing of the hydrogen amount is not a big problem.
2. The lowering of the hydrogen concentration is more difficult, since hydrogen is consumed practically not at all in the reaction. Nowadays the most common method of lowering the concentration of hydrogen in the reactor is to increase the exit stream of the accumulating inerts close to the maximum capacity of the gas removal system. The gas to be removed is directed, for example, to the flare for burning or as feed to cracking. When the amount of hydrogen of a relatively low concentration is being reduced, the gases exiting together with hydrogen will cause a great financial loss to the polyolefin plant.
Although the grade change situation is to be taken into account primarily in the operating of the reactors, it is clear that the concentration of the gas mixture exiting from the reactors together with the reactor product polymer varies. This variation complicates the planning and use of the gas separation unit.
As is evident from the foregoing, the accumulating light inert gases thus have to be removed from the polyolefin process. Usually it is done by directing, in connection with the gas separation distillation, the lightest components either to the flare, to the fuel gas network, or to the cracking unit feed. This exit gas stream also contains considerable amounts of components which should be recycled to the reactors, and therefore the conventional removal system is not a good solution. If the light gases are used as a fuel gas, the value of the stream is the combustion value of the components present in it. When they are directed to the flare, their value is virtually negative. If the light gases are directed as feed to the cracking unit, the cracking unit capacity, and thereby also the output of the plant, is lowered.
The object of the present invention is to eliminate the disadvantages associated with the prior art technology and to provide a completely new solution to the problem of treating the gas stream obtained from the polymerization of polyolefins.
The invention is based on the basic principle that hydrogen is separated from the gas stream of the gas-phase reactor by using a membrane system. The membranes are semipermeable films through which different gas molecules travel at different velocities. Thus it is possible to separate different components from one another. Separation based on membranes is in general advantageous in situations in which the substances treated have boiling points so low that the gas cannot be condensed by means of water or air, in which case this separation operation, important in the chemical process industry, becomes very expensive.
In connection with the present invention we have observed, surprisingly, that the placement of the membrane system in conjunction with distillation separators facilitates the separation of gases, and at the same time the operation of the entire polymerization process, and thereby also the profitability of the whole plant, is improved.
More specifically, the process according to the invention is characterized mainly by what is stated in the characterizing part of claim 1.
The invention comprises the use of a membrane separation apparatus for the separation of hydrogen from the hydrocarbon stream obtained from the polymerization reactor, the stream containing hydrogen and light, unreacted hydrocarbon compounds.
Considerable advantages are gained through the present invention. Thus, the separation of gases is facilitated, since by coupling the membrane system in conjunction with the distillation columns it is possible to carry out the distillation at a substantially higher top temperature. According to the invention the distillation column can be operated so that the temperature of the overhead product is 35xc2x0 C. This means that the condensing of the overhead product can be carried out, for example, by using cooling water or air. Usually the distillation column is operated so that the overhead product of the column is cooled by using a so-called refrigerating machine. The temperature of the top of the column is in this case typically approx. xe2x88x9220xc2x0 C. A prior art option such as this is quite expensive. If condensation with air or water is used, the uncondensed gas stream is very large, and its treatment is expensive and in all respects disadvantageous.
According to the invention, the separation of the gases also becomes more effective, since the use of the membrane system makes possible a better recovery of the monomer, comonomer and diluent. By using a membrane system it is possible effectively to remove the lightest gases, which would otherwise accumulate in the process.
The placement of the membrane system in conjunction with the distillation separators thus facilitates the separation of the gases and improves the operation of the entire polymerization process and thereby the profitability of the plant.
The invention will be examined below in greater detail with the help of a more detailed description and embodiment examples, with reference to the accompanying drawings.